Adding a vertex onto an angled edge in precise X position
You could subdivide the edge to add a vertex. Select the vertex and press GG to slide the vert. Make sure snapping is set to vertex and then click on the vert shown at 2 units above. This should line up the lower vertex on the angled line with the vertex above

The vertex above could be place at 2 units with snapping set to increment absolute grid.

 Answer above incorrect. New solution:

Ok this one bugged me as it should be possible to do this. I turned on edge length in the overlay



I subdivided the angled edge to create a vert and then GG and dragged the vert to the starting vert on the edge. I am working in m so in the image you can see 0m.



With the vert still selected I dragged it back along the angled edge whilst holding shift to get a more precise value. I moved the vert until it was at 0.5m



Should be able to use this technique for other stuff